Synthesis gas streams are gaseous streams being rich in carbon monoxide and hydrogen and further usually containing carbon dioxide, while also nitrogen, nitrogen-containing components (such as HCN and NH3) and steam may be present, as well as some other minor constituents. Synthesis gas streams are generally used for chemical processes, for example the preparation of hydrocarbons in a catalytic process, e.g. the so-called Fischer-Tropsch process.
Often, desulphurization of the feedstock used for the preparation of synthesis gas is difficult to achieve or incomplete and consequently unwanted contaminants such as sulphur compounds, especially H2S and COS, are still present in synthesis gas. The removal of these sulphur compounds is of considerable importance, because they may bind irreversibly on catalysts and cause sulphur poisoning. This results in a deactivated catalyst, which severely hampers the catalytic process. To this end, the removal of sulphur compounds to very low levels, in the ppb range, is required.
A process for producing a low sulphur synthesis gas suitable for high-performance Fischer-Tropsch catalysts is mentioned in U.S. Pat. No. 6,692,711. This process aims at deep desulphurization of the natural gas feedstock and teaches the use of ZnO and nickel in the natural gas. The use of ZnO only is well established for the removal of H2S in natural gas but it will not remove COS, as is observed in U.S. Pat. No. 6,692,711. The remedy applied in U.S. Pat. No. 6,692,711 is to use nickel to remove COS. However, when the aim is to remove COS from synthesis gas, the use of nickel is likely to result in unwanted side reactions such as the hydrogenation of CO and CO2. Nickel is a known methanation catalyst, see for example U.S. Pat. No. 4,888,131. The formation of methane is a highly exothermic reaction and thus very undesired. The process described in U.S. Pat. No. 6,692,711 can therefore not be applied to remove COS from a synthesis gas stream.
Processes for the removal of COS from a synthesis gas stream are known in the art. See for example H. M. Huisman, “The hydrolysis of carbonyl sulphide, carbon disulphide and hydrogen cyanide on a Titania catalyst”, 1994, Ph.D Thesis University of Utrecht, The Netherlands, ISBN 90-393-0534-X.
Another example is found is U.S. Pat. No. 6,322,763, in which a process is described wherein water in a wet scrubber is used to achieve hydrolysis of COS according to:COS+H2O<->H2S+CO2 
A disadvantage of the process is that it relies on a non-catalytic process and a “naturally occurring catalyst”. As an example alumina oxide as present in coal ash is used in the process described in U.S. Pat. No. 6,322,763. For other gasification processes such as oil-residue gasification and natural gas gasification, this catalyst will not be present and any particulate matter originating from the gasification step is usually removed by scrubbing prior to desulphurisation steps.
An even more important disadvantage of the process described in U.S. Pat. No. 6,322,763 is that the concentration of COS in the synthesis gas stays in the ppm range, which may result in catalyst poisoning in a catalytic conversion of the synthesis gas. The removal of COS to a very low concentration, typically in the ppbv range, cannot be achieved using a process as described in U.S. Pat. No. 6,322,763.
Especially when relatively high amounts of H2S are present in the synthesis stream, the hydrolysis of COS is difficult. H2S is a reactant and therefore would push back the hydrolysis of COS. The achievement of low concentrations of COS would be difficult.
Catalysts for the production of hydrocarbons from synthesis gas in a Fischer-Tropsch process are easily poisoned and can be permanently deactivated by sulphur. Even levels as low as 10 ppbv are unacceptable for a commercial hydrocarbon synthesis plant. Catalysts containing cobalt as a catalytically active element are particularly sensitive. Even levels as low as for example 5 ppbv are unacceptably high for a commercial hydrocarbon synthesis process wherein a cobalt catalyst is used. As the catalyst deactivates, hydrocarbon production decreases and the reactor has to be taken off line for catalyst replacement.
The removal of sulphur contaminants from synthesis gas, in particular the removal of COS which is more difficult to achieve compared to the removal of H2S, is therefore imperative in order to operate a productive hydrocarbon synthesis process.
Therefore, there is a need for a simple process enabling the removal of COS from synthesis gas to a low level, especially in the ppbv range, in the presence of H2S.